The present invention relates to an electric motor power steering apparatus, which allows an electric motor power to act upon a steering system to reduce the steering force of a driver. More particularly, it relates to an electric power steering apparatus, which not only provides stability of the steering system at a high vehicle speed range but also operates to require reduced steering assistance at a low vehicle speed range even at the time of failure of a vehicle speed sensor.
An electric power steering apparatus reduces (assists) steering force of a driver by the direct action of driving force of an electric motor steering. A vehicle having an electric power steering apparatus mounted thereon has a function of reducing the movement of the steering wheel whereby the driver needs not to operate the steering with strong force.
As one runs a vehicle at a higher speed, because of the reaction force from a road surface the steering wheel can move in a light manner as a rule. Therefore, at a time of running the vehicle at a high speed range, as large an assist steering torque is not required as is needed at the time of a low speed range. For this reason, the electric power steering apparatus controls in such a manner that the higher the vehicle speed based on the vehicle speed sensor, the lower the assist steering torque needed. Likewise, the steering force through a large assist steering torque at a low speed range makes it possible to enjoy steering operation in an easy manner, while decreasing the assist steering torque at a high speed thereby stabilizes the steering system.
FIG. 9 is the block configuration diagram of the electric system of a conventional electric power steering apparatus.
In this figure, a controller 112 is composed of a target current setting unit 121, a deviation calculator 122, a PDI controller 123, and a control signal output unit 124. The operation of the conventional electric power steering apparatus in FIG. 9 will now be described.
(1) The target current setting unit 121 inputs a steering torque signal from a steering torque sensor TS, which detects the steering torque of the driver and a vehicle speed signal from a vehicle speed sensor VS. According to a xe2x80x9csteering torque signal/vehicle speed signal-target currentxe2x80x9d map (not shown), which is increased when the steering. torque is increased and/or vehicle speed signal is decreased, the target current setting unit 121 decides the target current and outputs the detected target current to the deviation calculator 122. The target current setting unit 121 decides the target value of the current running though the electric motor.
(2) The deviation calculator 122 receives as input the target current and an electric motor current from an electric motor current sensor IS to calculate the deviation of them and outputs a deviation signal to the PID control unit 123.
(3) The PID controller 123 performs the procedure of P [proportion], I[integration], and D[differentiation] of the deviation signal and generates a PID signal, which is outputted to the control signal output unit 124.
(4) The control signal output unit 124 inputs the PID signal to generate an electric motor control signal composed of a direction signal and a PWM signal and outputs it to an electric motor driver 113.
(5) The electric motor driver 113 is composed of a gate driving circuit (not shown) and a bridge circuit having electric field effect transistors (not shown), and outputs an electric motor driving signal for driving an electric motor 108.
(6) The electric motor 108 is driven based on the electric motor driving signal, i.e., the steering torque signal and the vehicle speed signal; and the electric motor 108 applies an assist steering torque to a steering system (not shown).
By such a configuration, the steering operation is assisted by a large assist steering torque at a low vehicle speed range, making it possible to perform steering operation in a light manner. Conversely, at a high vehicle speed range, the assist steering torque becomes small to stabilize the steering system.
When the vehicle speed sensor fails, the assist steering torque cannot be controlled based on the vehicle speed. In this case, the electric power steering apparatus stops the control based on the vehicle speed and controls the assist steering torque at a constant level by setting a given dummy speed as a fail-safe action. For example, Japanese Patent Laid open publication No.60-80976 discloses a power steering apparatus which controls the steering force responding to the vehicle speed at the time of normal mode, and at the time of failure of the vehicle speed sensor, which generates control output so as to set the steering force (assist steering torque) at the middle value in a control range by a vehicle speed response. This is the same as the vehicle speed of the middle value being set as the dummy vehicle speed.
On the other hand, a vehicle having a xe2x80x9chydraulic power steering apparatusxe2x80x9d provided thereon, which varies an assist steering torque according to the revolution number of the engine until a vehicle speed signal is obtained and which varies the assist steering torque to respond to the vehicle speed at the time of obtaining the vehicle speed signal, has been known. According to our Japanese Patent Laid open publication No.9-86421, as a fail-safe measure dealing with the situation where no vehicle speed signal can be obtained in xe2x80x9celectric power steering apparatusxe2x80x9d due to any cause, a technique has been taught which varies the assist steering torque according to the revolution number of the engine until a vehicle speed signal is obtained, and switches the control of the assist steering torque to respond to the vehicle speed at the time of obtaining the vehicle speed signal. For example, the situation where the vehicle speed signal V in FIG. 9 is switched to the signal of the revolution number of the engine.
However, if a small value is taken as the dummy vehicle speed, although a light steering operation can be performed at a low vehicle speed range with a large assist steering torque, the movement of the steering wheel becomes too light, thereby lacking the stability of the steering system at a high vehicle speed range. In contrast, if a large value is taken as the dummy vehicle speed, the steering system is stabilized at a high vehicle speed range due to the small assist steering torque, but a strong force for steering operation must be applied at a low vehicle speed range. The steering force (assist steering torque) fixed at the middle value in the control range (dummy vehicle speed being set at a middle value) in order to respond to the vehicle speed as mentioned in Japanese Laid open publication No. 60-80976 leads to a lackadaisical result in terms of the facts that the assist steering torque becomes insufficient and steering operation becomes heavy at a low vehicle speed range, while the assist steering torque becomes in excess and stability of the steering system becomes insufficient at a high vehicle speed.
Furthermore, in the situation where the assist steering torque is varied according to the revolution number of the engine when no vehicle speed signal is obtained, the relation between the revolution number and the vehicle speed depends upon the gear ratio of a transmission (gear position). The change in the revolution number of the, engine totally has a relation with the vehicle speed, but sometimes occurs in a sharp manner in comparison with the change in the vehicle speed. Specifically, although the change in the revolution number of the engine is associated with the vehicle speed as a whole, the revolution number of the engine is sometimes changed irrelevant to the change in the vehicle speed. Consequently, such a configuration is not preferable because the change in the revolution number of the engine sensitively irrelevant to the vehicle speed has an affect upon the magnitude of the assist steering torque, changing steering feeling for the worse.
This invention has been made to solve the above problems, and it is, therefore, a main object of the invention to provide an electric power steering apparatus in which light steering operation is available at a low vehicle speed range and stability of the steering system can be obtained at a high vehicle speed range even if the vehicle speed sensor fails; and further steering feeling can be obtained.
The present invention which attains the object concerns an electric power steering apparatus comprising: an electric motor which applies an assist steering torque to the steering system of a vehicle; a steering torque sensor which detects the manual steering torque of said steering system; a vehicle speed sensor which detects the vehicle speed of said vehicle; a revolution number sensor for said electric motor which detects the revolution number of said electric motor; a controller which generates a control signal for controlling said electric motor: and a driver which generates a driving signal for driving said electric motor based on said control signal, said controller possessing: a reference torque signal setting unit which generates a reference signal which becomes high when the detected signal becomes high based on at least the signal detected from said steering torque sensor; a correction torque signal setting unit which generates a correction torque signal, which becomes high when the detected signal becomes high based on at least the signals detected from said revolution number sensor for the electric motor and from the vehicle speed sensor; and a target torque signal calculator which subtracts said correction torque signal from said reference torque signal to output a target torque signal to said electric motor; said controller also making said correction torque signal high when failure of the vehicle speed sensor is detected.
In this configuration, a large damping correction of the assist steering torque generated from the electric motor is made by making the correction torque signal high according to higher speed of the electric motor and vehicle speed at the time of normal mode when the electric motor (i.e., steering wheel) is revolved and the assist amount is reduced. Furthermore, the damping correction of the assist steering torque generated from the electric motor is made large by making the correction torque signal high regardless of an actual vehicle speed at the time of failure of the vehicle speed sensor.
Here, a reference torque signal setting unit generates a high reference torque signal for strong steering operation so that a large steering torque is generated (it is often seen at the low vehicle speed range). Therefore, a steering operation of a driver is greatly assisted by a larger assist steering torque even if the correction torque signal heightened due to vehicle speed sensor failure is subtracted from the high reference torque signal at the time of failure of the vehicle speed. Particularly, the driver is greatly assisted by the larger assist steering torque since the correction torque signal. becomes low for steering operation with the slow speed of the electric motor. Specifically, according to the configuration of the present invention, steering operation as seen at the low vehicle speed range is greatly assisted even if the vehicle speed sensor fails.
In contrast, the reference torque signal set unit generates a low reference torque signal for a weak steering operation so that a large torque is not generated (operation is often seen at the high vehicle speed range). Therefore, only a small assist steering torque is generated (generation of the assist steering torque in an opposite direction is also included) since a correction torque signal heightened due to failure of the vehicle speed sensor is subtracted from the low reference torque signal at the time of failure of the vehicle speed sensor. Therefore, a large assist steering torque is not generated and stability of the steering system (high speed stability) is secured even if such weak steering operation is performed at the high vehicle speed range at the time of failure of the vehicle speed sensor. Particularly, the assist steering torque becomes small for a steering operation in which the steering torque is small and the speed of the electric motor is fast. That is, according to the configuration of the present invention, the assist steering torque acts such that the steering system is stable even if a fast steering operation is performed at the high vehicle speed range.
Note that the term xe2x80x9cdamping correctionxe2x80x9d means that the correction of a signal is carried out in a direction opposing to the rotation direction of the steering wheel. Therefore, when the direction of steering torque and the rotating direction of the steering wheel are the same such as at the time of normal steering, the damping is subtraction correction. However, when the direction of steering torque is a direction opposed to the rotation of the steering wheel so that the steering wheel returns to a neutral position (self aligning torque etc.), the damping becomes an addition correction.
Take note that there are means to multiply the coefficient which increases the correction torque signal as a means to make the correction torque signal high and a means to input the high vehicle speed to the correction torque signal setting unit as a dummy vehicle speed as described in the embodiment of the invention.
According to the present invention, there is a configuration wherein said reference torque signal setting unit generates a reference torque signal based on the signal detected from the vehicle speed sensor in addition to the signal detected from the steering torque sensor under the condition that a small reference torque signal is generated in the case where the vehicle speed sensor detects a large signal in comparison with the case of a small signal being detected, and said controller makes said reference torque signal high when said vehicle speed sensor failure detection unit detects failure of the vehicle speed sensor.
According to this configuration, the reference torque signal setting unit (steering torque and vehicle speed response type) outputs a higher reference torque signal according to a lower vehicle speed signal if the steering torque signal is the same. Since the reference torque signal becomes high when failure of the vehicle speed sensor is detected, operation is preferably assisted, especially, at the low vehicle speed area at which an actual vehicle speed is low.
In each configuration, a storage device to store the previous vehicle speed and a deceleration detector which detects the degree of deceleration with reference to the current and previous vehicle speeds are provided and it is preferable to detect that failure arises when the previous vehicle speed is not less than a predetermined vehicle speed (20 km/h) and the degree of deceleration is not less than a predetermined value (2.5 G). This is because of deceleration having no possibility in braking operation. In is each configuration, a detector which detects the revolution number of the engine and a timer are provided and it is preferable to detect that failure arises when a value is not more than a predetermined revolution number of the engine (200 rpm) and the state of a vehicle speed=0 km/h is continued not less than a predetermined time (180 seconds). This is because there is no possibility of such a racing state in usual.
Further, it is preferable that the controller makes the reference torque signal and/or correction torque signal gradually higher by so called fadeout control in each configuration when it makes the reference torque signal and/or correction torque signal high at the time of failure of the vehicle speed sensor. This is because there is insecurity being given to a driver. Various methods are suggested as for the method of this fadeout.
According to the present invention, there is provided an electric power steering apparatus comprising: an electric motor which applies an assist steering torque to the steering system of a vehicle; a steering torque sensor which detects the manual steering torque of said steering system; a vehicle speed sensor which detects the vehicle speed of said vehicle; a revolution number sensor for said electric motor which detects the revolution number of said electric motor; a controller which generates a control signal for controlling said electric motor; and a driver which generates a driving signal for driving said electric motor based on said control signal. The controller possesses a reference torque signal setting unit which generates a reference signal which becomes high when the detected signal becomes high based on at least the signal detected from the steering torque sensor; a correction torque signal setting unit which generates a correction torque signal, which becomes high when the detected signal becomes high based on at least the signals detected from the revolution number sensor for the electric motor and from the vehicle speed sensor; and a target torque signal calculator which subtracts the correction torque signal from the reference torque signal to output a target torque signal to the electric motor. Also, in this embodiment, the power steering apparatus possesses a revolution number sensor for the engine which detects the revolution number of the engine and a vehicle speed 0 detection unit which detects that the vehicle speed is zero, and is configured so that the controller makes the correction torque signal high according to the signal detected from the revolution number sensor for the engine, when the vehicle speed 0 detection unit detects that the vehicle speed is zero.
In this configuration, a large damping correction of the assist steering torque generated from the electric motor is carried out by making the correction torque signal high according to higher speed of the electric motor and vehicle speed at the time of normal mode when the electric motor (i.e., steering wheel) is revolved and the assist amount is reduced. Furthermore, the controller makes the correction torque signal change according to at least the electric motor speed signal and the signal of the revolution number of the engine responding to the revolution number of the engine instead of the vehicle speed when the assist steering torque is dampxe2x80x94corrected. Take note that failure of the vehicle speed sensor is disconnection failure so as to output vehicle speed 0. Therefore, a countermeasure for the vehicle speed sensor can be taken without judgment whether or not the vehicle speed sensor fails in this configuration.
Here, a reference torque signal set unit generates a high reference torque signal for strong steering operation so as to generate a large steering torque with the decreased revolution number of the engine in the case where the degree of the signal detected from the vehicle speed sensor is vehicle speed. At the same time, the correction torque signal becomes low by the decreased revolution number of the engine (it is often seen at the low vehicle speed range). Therefore, steering operation often seen at the low vehicle speed range is greatly assisted by a large assist steering torque even if the detection signal from the vehicle speed sensor is vehicle speed 0 (e.g., even if the vehicle speed sensor fails). The driver can quickly perform the steering operation adequately without insecurity against steering operation since the revolution number of the engine is decreased at the low vehicle speed area (correction torque signal is low) even if steering operation at a high revolution number of the electric motor (sudden steering) is performed. That is, according to the configuration of this embodiment, the steering operation as seen at the low vehicle speed range is adequately assisted even if the signal detected from the vehicle speed sensor is vehicle speed 0.
Similarly, the reference torque signal setting unit generates a small reference torque signal for a weak steering operation so as to generate only a small steering torque with the increased revolution number of the engine (seen at the high vehicle speed range) when the signal detected from the vehicle speed sensor is vehicle speed 0. At the same time, the correction torque signal becomes high for a decreased revolution number of the engine. Therefore, the stability of the steering system (high vehicle speed stability) is secured without generating the large assist steering torque even if the signal detected from the vehicle speed sensor is vehicle speed 0 for a steering operation seen at the high vehicle speed range (e.g., vehicle speed sensor fails). Particularly, the assist steering torque becomes small for a steering operation at a small steering torque and at a high revolution number of the electric motor. That is, the assist torque acts so that the steering system is stable even if the signal detected from the vehicle speed sensor is vehicle speed 0 for a steering operation at the high vehicle speed range according to this embodiment.
Note that the term xe2x80x9cdamping correctionxe2x80x9d means that the correction of a signal in a direction opposed to the rotation direction of the steering wheel is made. Therefore, when the direction of steering torque and the rotating direction of the steering wheel are the same such as at the time of normal steering, the damping is subtraction correction. However, when the direction of steering torque is a direction opposed to the revolution of the steering wheel so that the steering wheel returns to a neutral position (self aligning torque etc.), the damping becomes an addition correction. Damping correction of the present invention greatly differs from the prior art. Influence of change in the revolution number of the engine not associated with the vehicle speed, which is a problem in the prior art, is less at the time of generating the correction torque signal and outputting the target torque signal. Therefore, a target value (target current) for the electric motor is set based on the steering torque signal and a signal of revolution number of the engine even if a negative portion of the response of revolution number of the engine is dampened and the steering feeling does not become worse.
According to the present invention, there is provided an electric power steering apparatus comprising: an electric motor which applies an assist steering torque to the steering system of a vehicle; a steering torque sensor which detects the manual steering torque of the steering system; a vehicle speed sensor which detects the vehicle speed of the vehicle; a revolution number sensor for the electric motor which detects the revolution number of the electric motor; a controller which generates a control signal for controlling the electric motor; and a driver which generates a driving signal for driving the electric motor based on the control signal. The controller possesses a reference torque signal setting unit which generates a reference signal which becomes high when the detected signal becomes high based on at least the signal detected from the steering torque sensor; a correction torque signal setting unit which generates a correction torque signal, which becomes high when the detected signal becomes high based on at least the signals detected from the revolution number sensor for the electric motor and from the vehicle speed sensor; and a target torque signal calculator which subtracts the correction torque signal from the reference torque signal to output a target torque signal to the electric motor. Also, in this embodiment, the power steering apparatus possesses a revolution number sensor for the engine which detects the revolution number of the engine and a vehicle speed sensor failure detection unit which detects failure of the vehicle speed sensor. The controller is configured to make the correction torque signal high according to the signal detected from the revolution number sensor for the engine, when the vehicle speed sensor failure detection unit detects failure of the vehicle speed sensor.
In this configuration, a large damping correction of the assist steering torque generated from the electric motor is carried out by making the correction torque signal high according to higher speed of the electric motor and vehicle speed at the time of normal mode including actual vehicle speed 0 when the electric motor (i.e., steering wheel) is revolved and the assist amount is reduced. Furthermore, the damping correction of the assist steering torque generated from the electric motor generates is carried out by making the correction torque signal change according to at least the electric motor speed signal and the signal of the revolution number of the engine responding to the revolution number of the engine instead of the vehicle speed when the assist steering torque is damp-corrected. In this configuration, response of the revolution number of the engine is controlled only at the time that the vehicle speed sensor fails. Therefore, the operation of this configuration is adequately assisted by large assist torque at the low vehicle speed range and steering system becomes stable since the large assist steering torque is not generated at the high vehicle speed range. Also, the steering feeling does not become worse.
Furthermore, in each configuration, a storage device which stores the previous vehicle speed and a damping speed detector which detects the degree of deceleration with reference to the current and previous vehicle speeds are provided in the vehicle speed sensor failure detection unit, and it is preferable that the vehicle speed sensor failure detection unit detects that failure arises when the previous vehicle speed is not less than a predetermined vehicle speed (20 km/h) and the degree of deceleration is not less then a predetermined value (2.5 G). This is because there is no possibility of deceleration in usual braking operation. In each configuration, a detector of the revolution number of the engine and a timer are provided in the vehicle speed sensor failure detection unit and it is preferable the vehicle speed sensor failure detection unit detects that failure when the case where the revolution number of the engine is not less than a predetermined value of the revolution number of the engine (2000 rpm) and a vehicle speed is 0 km/h is continued not less than a predetermined time (180 seconds). This is because there is usually no possibility of such racing state.
Further, in each configuration, it is preferable that the controller makes the reference torque signal and/or correction torque signal gradually higher by fadeout control when it makes the correction torque signal high according to the signal detected from the sensor of the revolution number of the engine by switching the correction torque signal into the signal detected from the vehicle speed sensor at the time of failure of the vehicle speed sensor. The driver does not then become insecure. Various methods are suggested as for the method of this fadeout.